U.S. patent number 6,323,804 [Application Number 09/587,866] was granted by the patent office on 2001-11-27 for method and apparatus for gps time determination.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Daniel T. Buhmann, Michael D. Kotzin, Christopher N. Kurby.
United States Patent |
6,323,804 |
Kurby , et al. |
November 27, 2001 |
Method and apparatus for GPS time determination
Abstract
A system for rapidly acquiring a time reference for the location
determination of a wireless communication device. The system
includes a wireless communication device (200), a GPS satellite
(202) and a communication satellite (208). The method comprises the
steps of acquiring a communication satellite signal (210) and using
the finite frame time to establish a course time reference. Once
the course time reference is established, an absolute device time
is determined by the wireless communication device, which as a
result is synchronized to the absolute time of the communication
satellite (208) to within ten milliseconds. The absolute device
time is then used to synchronize the GPS portion of the wireless
communication device with the GPS satellite system.
Inventors: |
Kurby; Christopher N.
(Elmhurst, IL), Kotzin; Michael D. (Buffalo Grove, IL),
Buhmann; Daniel T. (Hainesville, IL) |
Assignee: |
Motorola, Inc. (Schaumburg,
IL)
|
Family
ID: |
24351511 |
Appl.
No.: |
09/587,866 |
Filed: |
June 6, 2000 |
Current U.S.
Class: |
342/357.64;
342/104; 342/115; 342/352; 342/354; 342/457; 342/46; 342/50;
370/316; 370/330; 370/337; 370/344; 370/478; 370/480; 375/299;
375/347; 455/446; 455/456.1; 700/200; 700/201; 700/202; 700/203;
700/204; 700/205; 700/206; 700/207; 700/208; 700/209; 700/210;
700/211; 700/212; 700/213; 700/214; 700/215 |
Current CPC
Class: |
H04B
7/2125 (20130101) |
Current International
Class: |
H04B
7/212 (20060101); G01S 1/00 (20060101); G01S
005/00 (); G01S 013/00 (); H04B 007/00 (); H04B
011/00 (); H04B 015/00 () |
Field of
Search: |
;342/357.09,357.01,357.08,357.06,104,115,50,46,457,357,352,354,357.12
;701/200-215 ;455/456,12.1,13.4,428,446,62,63
;370/330,316,337,344,478,480 ;375/347,436,202,299 ;340/982,989 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Cuchlinski, Jr.; William A.
Assistant Examiner: Mancho; Ronnie
Attorney, Agent or Firm: Collopy; Daniel R.
Claims
What is claimed is:
1. A portable communication device comprising:
a first satellite receiver for receiving a first satellite signal,
said first satellite signal, in the form of a beam having a beam
center, includes a first satellite data stream, formatted into
finite time frames sequentially transmitted, said finite time
frames include an absolute satellite time and a frame time
associated with said absolute satellite time, a first satellite
ephemeris data, and beam center data;
a microprocessor coupled to said first satellite receiver for
extracting said absolute satellite time from said first satellite
data stream to generate a absolute device time that is synchronized
with said absolute satellite time,
wherein said absolute satellite time and said absolute device time
have a time reference ambiguity associated therewith; and
a Global Positioning System (GPS) receiver coupled to said
microprocessor for receiving said absolute device time from said
microprocessor and using said absolute device time to synchronize
said GPS receiver with at least three GPS satellites and further
calculating a location therefrom.
2. The device as in claim 1 wherein said first satellite frame time
is broadcast by said first satellite at least every 540
milliseconds.
3. The device as in claim 1 wherein said microprocessor selects a
first satellite signal from a plurality of first satellite signals
by determining the relative strength of each individual first
satellite signal and selecting the first satellite signal with the
highest power level.
4. The device as in claim 1 wherein said microprocessor uses said
frame time as a course time reference in the GPS system.
5. The device as in claim 1 wherein said first satellite receiver
selects a first satellite signal based on the last known position
of said device.
6. The device as in claim 4 wherein the time reference ambiguity
between said absolute satellite time and said absolute device time
is no more than 10 milliseconds.
7. The device as in claim 1 further comprising a portable
communication transmitter, said communication transmitter for
transmitting said GPS location to a communication system base
station,
wherein said communication system base station calculates a precise
location of said portable communication device using said absolute
satellite time.
8. A method for determining the position of a communication device,
said method comprising:
acquiring a first satellite signal with a first satellite receiver,
said first satellite signal having a first satellite frame time
data, an absolute satellite time, and a satellite beam center;
extracting said first satellite frame time data from said first
satellite signal;
calculating an absolute device time based on said first satellite
frame time data and a propagation time estimate;
synchronizing a GPS receiver using said absolute device time
data;
determining a precise location of said communication device with
said GPS; and
transmitting said precise location from a portable communication
transmitter portion of said device.
9. The method as in claim 6 wherein said first satellite receiver
selects a first satellite signal by selecting from a plurality of
first satellite signals, the signal with the highest relative
signal level,
wherein the absolute time error of said first satellite signal is
less than 3.2 milliseconds.
10. The method as in claim 6 wherein said first satellite receiver
selects a first satellite signal based on the last known position
of said device,
wherein the absolute time error of said first satellite signal is
less than 1.1 milliseconds.
11. A portable communication device comprising:
a satellite receiver for receiving a satellite signal, said
satellite signal includes satellite data, formatted into finite
time frames sequentially transmitted, said finite time frames
include an absolute satellite time and a frame time associated with
said absolute satellite time; and
a microprocessor coupled to said satellite receiver for extracting
said absolute satellite time from said satellite data to generate a
device absolute time that is synchronized with said absolute
satellite time.
12. The device as in claim 11 further comprising a Global
Positioning System (GPS) receiver coupled to said microprocessor
for receiving said absolute device time from said microprocessor
for synchronizing to said GPS, and calculating location information
therefrom.
13. The device as in claim 11 wherein said satellite frame time is
broadcast by said satellite at least every 540 milliseconds.
14. The device as in claim 11 wherein said cellular transmitter
receives said location information and transmits said location
information to a cellular base station.
15. The device as in claim 11 wherein said satellite receiver of
said wireless communication device selects a satellite signal from
a plurality of satellite signals by determining the relative
strength of each individual satellite signal and selecting the
satellite signal with the highest power level, and
wherein said satellite receiver further determines a course
location estimate.
16. The device as in claim 11 wherein said satellite receiver
selects an satellite signal based on the last known position of
said device, and
wherein said satellite receiver determines a course location
estimate.
17. The device as in claim 13 wherein the absolute time error of
said first satellite communication system signal is at most 3.2
milliseconds.
18. The device as in claim 14 wherein the absolute time error of
said first satellite communication signal is at most 1.1
milliseconds.
19. The device as in claim 11 wherein said satellite receiver
determines a course location estimate using a passive geo method.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to a method and apparatus
for determining a time reference in a cellular communication
system. More particularly the present invention relates to
expeditiously synchronizing a Global Positioning System (GPS)
receiver with its associated GPS satellites and as a result
decreasing the device location acquisition rate.
Traditionally, wireless communication devices have functioned
solely as a communication device. However, in light of emergency
situations and the increased need for location based services in
general, the need to accurately locate the subscriber unit (SU)
within a given geographical area has become a necessity. Calls
originating from a SU under an emergency context can cause a
problem for the dispatcher, as the individual using the SU often
does not know his exact location. This can have a substantial
effect on the time it takes emergency personnel to reach the
distressed individual and in life and death situations, time may
make a substantial difference. Other location based services,
although not as critical as in emergency dispatch, can benefit from
the capability to accurately locate the SU.
It is desirable to obtain and communicate physical locations of
wireless communication devices within a system, such as
radiotelephone handsets within a cellular communication system. In
addition, the United States Federal Communications Commission (FCC)
has required that cellular communication handsets must be
geographically locatable by the year 2001. This capability is
desirable for emergency systems such as Enhanced 911 (E911). The
FCC requires stringent accuracy and availability performance
objectives and demands that cellular communication handsets be
locatable within 100 meters 67% of the time for network based
solutions and within 50 meters 67% of the time for handset based
solutions.
Current generations of cellular communication devices have only
limited SU location determination capability. In one technique, the
position of the SU is determined by monitoring SU transmissions at
several base stations. From time of arrival measurements, the SU's
position can be calculated. However, the precision of this
technique is limited and, at times, may be insufficient to meet FCC
requirements.
Another method and apparatus for determining the location of a SU
is to incorporate a Global Positioning System (GPS) receiver into
the SU. The GPS receiver is capable of receiving signals from a GPS
satellite constellation in a high earth orbit. The GPS receiver is
coupled to the microprocessor of the SU and provides location data
thereto. This location data may then be transmitted over the
cellular communication system from the SU to a base station and
then further onto the emergency service requesting the
information.
Location determination for GPS is based on triangulation
calculations measuring the distance the signal travels from the GPS
satellite to the GPS receiver. This method requires the GPS
receiver in the SU to have the same time reference as the GPS
satellite sending the signal. The signal sent by the GPS satellite
includes its time reference along with other information including
satellite ephemeris information. Having the same time reference
allows the SU to determine how long the signal traveled in time
from the GPS satellite to the GPS receiver. Since the signal
travels at the speed of light, the distance traveled can be
calculated knowing the time it took the signal to reach the SU.
However, the SU must initially have a absolute device time in order
to rapidly synchronize with the GPS system. This is exacerbated
with infrastructure aided GPS systems which use a one second
integration time. The absolute device time must be within 10
milliseconds for the GPS calculations to be effective in
determining the device location. Acquisition based on time
reference acquisition can take an extensive amount of time as the
receiver must search all possible code phases. This is because
there are multiple satellites sending data at the same time, which
are constantly moving at a high rate of speed, in addition to
atmospheric aberrations that disrupt the signal as it travels from
the satellite down to the SU. This acquisition time may be in terms
of minutes to establish the location of the device. In an emergency
situation, this time to acquire the time reference and subsequently
the location information is unacceptable as it leads to large
delays in dispatching the appropriate help and in life and death
situations this time may be critical.
Finally, the Iridium.TM. satellite communication system has the
capability to locate a SU to within 10 kilometers using a passive
geo method for determining the SU location without a GPS receiver.
This system too, is clearly unacceptable in terms of accuracy
required for the location based services.
Accordingly, a system is needed to improve the time to determine
the location of a SU. This is the case not only for emergency
situations but for location based services which can improve
service with the capability to receive expedited and accurate
location information.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a cellular radiotelephone system in
accordance with the preferred embodiment.
FIG. 2 is a satellite beam footprint in accordance with the
preferred embodiment.
FIG. 3 is a representation of the data stream showing the low orbit
satellite data frames in accordance with the preferred
embodiment.
FIG. 4 is a flow chart showing the acquisition of the time
reference from the low orbit satellite in accordance with the
preferred embodiment.
FIG. 5 is a representation of a satellite constellation in
accordance with the preferred embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An improved system and method is described for establishing a time
reference in a wireless communication device. The system comprises
the novel integration of a GPS receiver and a communication
satellite receiver into a wireless communication device to rapidly
acquire a time reference used by the GPS system to determine the
device location. Acquiring an accurate geographical position in a
very short time is desirable for Location Based Services (LBS) and
essential for Enhanced 911 (E911) systems in order for these
systems to be effective.
Turning to FIG. 1, a block diagram of a wireless communication
device and more particularly a cellular radiotelephone system in
accordance with the preferred embodiment is shown. In addition to
typical components of a cellular radiotelephone, the communication
device includes a first receiver 136 and a second receiver 138. In
the preferred embodiment of the present invention, the first
receiver 136 is for receiving satellite communication signals and
the second satellite receiver 138 is for receiving GPS signals. A
multiband antenna 134 is coupled to the first receiver 136, the
second receiver 138 and a wireless communication device receiver
132.
In the preferred embodiment a frame generator ASIC 102, such as a
CMOS ASIC available from Motorola, Inc. and a microprocessor 104,
such as a 68HC11 microprocessor also available from Motorola, Inc.,
combine to generate the necessary communication protocol for
operating in a cellular system. Microprocessor 104 uses memory 106
comprising RAM 108, EEPROM 110, and ROM 112, preferably
consolidated in one package 114, to execute the steps necessary to
generate the protocol and to perform other functions for the
wireless communication device, such as writing to a display 116,
accepting information from a keypad 118, or controlling a frequency
synthesizer 130 and controlling a cellular communication receiver
132, a first satellite receiver 136 for receiving GPS signals, and
a second satellite receiver 138 for receiving satellite
communication signals. ASIC 104 processes audio transformed by
audio circuitry 124 from a microphone 122 and to a speaker 126.
In the preferred embodiment of the present invention, a wireless
communication device operates in a cellular radiotelephone
communication system. This system serves the general public by
providing wireless communication between multiple users
geographically separated. In the present invention, the cellular
system may utilize current second generation (2G) cellular systems
using one or more of the following technologies: CDMA, TDMA, GSM,
or iden, or a third generation (3G) system incorporating cdma2000
or WCDMA or the like. In the preferred embodiment of the present
invention the SU transmits voice communications and positional
information over a 2G cellular communication system. The positional
information received at the base station (BS) can then be processed
by the BS and directed to the appropriate source.
A pictorial representation of the wireless communication system of
the preferred embodiment of the present invention is shown in FIG.
2. The overall system incorporates a wireless communication device
200, a GPS satellite 202, a communication satellite 208 and a
wireless communication system base station 216. The GPS receiver
incorporated into the wireless communication device is attuned to
receive Navastar GPS radio frequency (RF) signals at 1575 Mhz. The
plurality of satellites that make up the GPS system are referred to
as a constellation as shown in FIG. 5. The GPS satellite 202,
representing one in the GPS constellation, transmits GPS data
necessary to the operation of the GPS system. The GPS data
transmitted includes at least a satellite time reference and
ephemeris data giving satellite positional information specific to
the individual satellite. This data is used in accordance with the
operation of the GPS system to allow the SU 200 to triangulate the
position thereof. Efficient operation of the GPS system requires
that the time ambiguity between the absolute satellite time and the
absolute device time be less than 10 milliseconds. This allows the
GPS satellites to be rapidly acquired.
The communication satellite receiver, also shown in FIG. 2 is for
receiving communication signals from a satellite communication
system, such as the Globalstar.TM. or Teledesic.TM. satellite
communication systems. The communication satellite may also include
a plurality of satellites in their own respective constellation.
Each satellite, within the satellite constellation has at least one
transmission signal directed to the earth forming a highly
predictable reception zone on the earth's surface. The signal
contains communication information as well as an absolute time
reference used to synchronize the digital communication
information.
The communication satellite signal propagation path forms a beam as
it travels from the satellite to the earth thereby forming an area
on the earth. Each satellite transmits at least one RF beam, each
RF beam having the capability to communicate with multiple users
simultaneously. The RF beam has an RF beam center and the
geographical coordinates of the RF beam center are known. As the RF
beam center travels across the earth's surface, the coordinates of
the RF beam center are transmitted within the communication
satellite signal. The device on the receiving end can receive this
data allowing the device to determine which beam center is closest
for best reception.
If the wireless communication device is within a 300 km distance of
its last known location, the device will select a best beam center
by the last known GPS location stored. The time ambiguity as a
result will be within +/-1.1 milliseconds because the location can
be used to calculate the signal propagation time delay using the
communication satellites known latitude and longitude transmitted
in the communication satellite data signal. If the SU finds the
beam center within 300 km, of the last known location, the SU will
use this positional information to prime the GPS search. If a beam
center is not found within 300 km of the last known position, the
beam center with the highest power will be used to prime the GPS
search. Selecting the best beam center by the highest power may
take an average of eight seconds, whereas selection is immediate if
the is a beam center is located within 300 km of the last known
location.
The satellite communication data signal 210, transmitted by the
satellite communication satellite 208, is transmitted at a power
relatively greater than GPS transmission power, allowing faster
acquisition of the satellite communication data signal by the
communication satellite receiver of the wireless communication
device. Once the communication satellite beam has been selected,
the SU will read the frame time. The satellite communication data
signal 210 is shown in FIG. 3. The frame time is broadcast every
540 milliseconds in the preferred embodiment of the present
invention allowing the SU to determine absolute time to within plus
or minus the propagation time error. Within each time frame are
time slots U1-4, and D1-4, 306-320. The beginning of each time
frame is distinguished by a ring slot 304. This is a unique slot
containing the latitude and longitude of the communication
satellite. This information allows synchronization of the SU with
the satellite by identifying each frame symbol as the start of each
frame. In the case of the preferred embodiment of the present
invention the time ambiguity will be at most +/-3.2 milliseconds.
Either method of selecting the satellite and respective signal
allows for a time ambiguity of less than 10 milliseconds as
required for the GPS to determine position within the accuracy
requirements.
The wireless communication device, when turned on or after losing
the signal for an extended period of time, will begin an
acquisition cycle to expeditiously acquire a GPS satellite signal
and time reference data therefrom in order to provide the time
reference data to the GPS system. Simultaneously, the satellite
communication receiver will search for a satellite communication
signal from which a time reference may also be obtained. The
satellite communication signals are generally stronger than a GPS
signal allowing faster acquisition of the satellite communication
signal and retrieval of time reference data. The flow chart in FIG.
4 shows the steps involved in rapid infrastructure aided
acquisition of the time reference data and the further
establishment of an absolute device time which is synchronized with
the absolute satellite time in the satellite communication system
and the GPS satellite which are both maintained by an atomic clock
in each individual satellite within each satellite system.
Step 400 in FIG. 4 begins the process and signifies when the
wireless communication device is turned on or when the GPS
synchronization is lost due to signal fading or other disruptions.
The device will begin to start a passive geo search 402 and
simultaneously compare the last known position 404 with the closest
current beam center from the satellite communication system. The
passive geo search 402 may take between nine seconds to forty five
minutes. This large variation in signal acquisition time drives the
need for infrastructure aided GPS, which reduces this signal
acquisition time substantially. If the passive geo search 402
completes its acquisition of at least three satellite positions
prior to the last know position comparison 406 step, then the last
known position comparison 406 step is terminated and the system
moves to step 412. If the passive geo acquisition 408 is not
complete then the device will search for the closest satellite
communication beam center based on power 410. Once the closest beam
center is located the wireless communication device will extract
the beam lookup table from the communication satellite signal. The
beam lookup table includes an estimated propagation error t.sub.p
associated with that satellite.
In step 412 the wireless device will use the satellite ephemeris
data to estimate the signal propagation time t.sub.p corresponding
to at least three satellites. Next, in step 414 the SU 200
calculates, the absolute device time from the position estimate or
last known position and the communication satellite ephemeris data.
Once the SU 200 has calculated the absolute device time 414 and
rough location rapidly, the GPS can synchronize the code division
multiple access (CDMA) pseudo noise codes and establish links with
at least three GPS satellites. Once the GPS portion of the
communication device has linked with at least three GPS satellites.
The location of the SU 200 can be determined through the techniques
used in the GPS system.
In an alternative embodiment of the present invention, the wireless
communication device will transmit GPS data collected thereby over
a communication system, such as a cellular communication system, to
a base station (BS). The BS will use the GPS data to calculate the
position of the device. The method may be utilized when the
subscriber unit is limited in space and weight and therefore can
not make the necessary positional calculations. The base station on
the other hand is not as limited by space or weight and retains the
necessary microprocessor and related system architecture to make
the positional calculations. The base station may track a plurality
of SU in the field and will therefore have the positional data in a
common location which makes access to the information unencumbered.
This will allow for fast calculations involving data from multiple
SU's efficient and achievable.
While the invention has been described in detail above, the
invention is not intended to be limited to the specific embodiments
as described. It is evident that those skilled in the art may now
make numerous uses, modifications of, and departures from the
specific embodiments described herein without departing from the
inventive concepts.
* * * * *